[Haskell-cafe] is it possible to implement Functor for ByteString and Text

[Christopher Done]

You could have
class IsWord8 a
instance IsWord8 Word8
instance (IsWord8 a) => Functor (ByteString a) where ...

It would be a legitimate instance.

On 28 February 2015 at 20:41, David Feuer <david.feuer at gmail.com> wrote:
> This is impossible. The type of fmap is
>
> fmap :: Functor f => (a->b) -> f a -> f b
>
> You can use a GADT to effectively restrict what a is, because the
> caller won't be able to provide a non-bottom f a if a has the wrong
> type. But the caller can choose absolutely any type for b, and there's
> nothing you can do about that.
>
> David
>
> On Sat, Feb 28, 2015 at 2:11 PM, silvio <silvio.frischi at gmail.com> wrote:
>> I have recently heard that some people want to burn bridges (introducing
>> Foldable and Traversable to Prelude) and I've been wondering if it was
>> possible somehow allow Text and Bytestring like containers to make use
>> of those functions. Something along the lines of
>>
>> import qualified Data.ByteString as BS
>>
>> newtype ByteString' a = ByteString' BS.ByteString
>>
>> type ByteString = ByteString' Word8
>>
>> instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString')
>>     where
>>         fmap f (ByteString' bs) = ByteString' $ BS.map f bs
>>
>>
>> Or if DataContexts worked as you would expect.
>>
>> newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString
>>
>> However I couldn't find a solution and I was just wondering if it is
>> possible.
>>
>> P.S. Using GADTS it does actually work for Foldable, but only because it
>> doesn't have to output any ByteStrings. It doesn't work for Functor for
>> instance.
>>
>> data ByteString' a where
>>         ByteString' :: BS.ByteString -> ByteString' Word8
>>
>> type ByteString = ByteString' Word8
>>
>> instance Foldable ByteString' where
>>         foldr f ini (ByteString' bs) = BS.foldr f ini bs
>>
>>
>> Silvio
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